This invention relates to analog switching apparatus and more particularly to a gallium arsenide MOSFET switching cell exhibiting wide linear dynamic range.
MOS devices make excellent switches and choppers for many applications such as modulators, demodulators, sample and hold systems, mixers, multiplexers, gating and similar circuits. Several important characteristics of the MOSFET make it almost ideal for these applications. One advantage is that there is no inherent offset voltage associated with MOSFETs as there is with two junction transistors, since the conduction path between drain and source is predominantly resistive. That is the conduction channel is either depleted or enhanced by controlling an induced field.
The DC gate input impedance of a MOSFET is also extremely high and requires little power since the gate is essentially a capacitor. The impedance is determined by the properties of the insulation layer of the gate. An important advantage of the MOS device when used as a switch is the exceptionally high ratio of OFF resistance to ON resistance of the drain source channel. The resistance can be as low as 50 ohms in the ON condition and higher than thousands of megohms in the OFF condition. However, a zero ON resistance is not possible with MOS devices which may be a main disadvantage in some applications.
A main disadvantage of MOS devices is the capacitance between the gate and drain, and the gate and source. The capacitance feeds through part of the gate control voltage to the signal path and is detrimental to high frequency signal isolation, also imposing a limitation on response times.
In any event, the MOS device has been utilized in the prior art in many applications as a switching device or a chopper. See for example a text entitled "Manual for MOS Users" by John D. Link, published by Reston Publishing Company Inc., a Prentice Hall company (1975). See Chapter 4 entitled "Practical MOSFET Applications" pp. 192-209.
As will be explained in regard to the prior art, FET circuits display limited linear dynamic range and basically respond poorly for large signal amplitudes. The OFF isolation of many FET circuits is poor unless there are large differences between the input signal and the applied gate voltage and as such these devices require large magnitude voltage supplies in order to enable linear switching. Thus, for many of such circuits large signal application calls for building up a large gate voltage which when added to a large input signal may present a breakdown hazard.
Another problem with employing large gate voltages is associated with a large gate resistance which prevents the gate voltage from tracking the signal over a broadband of frequency ranges. This is due to the effect of large parasitic capacitances and gate associated capacitances which are present at the gate node. Poor tracking of gate voltage with signal voltage make insertion loss as well as OFF isolation frequency dependent. However, if the gate resistor is kept reasonably small than the control current has to be proportionally increased to maintain the large gate voltage. As a result, control circuit devices have to be scaled up. Thus it was discovered that during operation of many such circuits loading of the input and the output was substantial if the control circuit devices were subjected to large signal fluctuations.
It is therefore an object of the present invention to provide an improved FET analog switch cell which cell exhibits wide linear dynamic range and which avoids many of the above-noted problems associated with the prior art.
It is a further object to provide an analog switch cell employing MOSFETs or MESFETs which exhibits improved tracking of the gate voltage with signal voltage.
These and many other objects of the present invention will be described with reference made to the detailed description of the invention.